It is a mystery worthy of an "Indiana Jones" movie. A woman comes forward to tell historians that, according to family tradition, the oldest unopened time capsule in the world was lodged in the mouth of a golden lion statue atop Boston's Old State House.

Last month, the Bostonian Society got an inkling that it was true. Amid renovations on the Old State House, the capsule was discovered.

But it was not opened until Thursday at a private ceremony in front of reporters in Woburn, Mass. The box, it turned out, was full, with a red hardcover book sitting on top. The Bostonian Society decided not to take out the book at the ceremony. Instead, historians will examine all the contents of the capsule soon in a more controlled environment.

The Bostonian Society, which preserves historical Boston artifacts and maintains the city's Old State House, was alerted to the possibility of the time capsule three years ago. The tipoff came from a descendant of a carpenter who worked on the Old State House in 1901.

Later, historians discovered a Boston Globe article from 1901 that suggested campaign buttons and sealed letters, including one from the mayor, were inside, as well as a family history of the carpenter, according to Boston.com.

The society found the capsule after the lion statue was removed for refurbishment. A camera was inserted into a crack in the statue, revealing the small box.

But Boston's rough winter conditions make it difficult for paper items to hold up over the years, says Nathaniel Sheidley, director of public history at the Bostonian Society.

"When there's a Nor'easter you could just see the snow and sleet drumming against the lion," he says.

The box's contents hold symbolic significance, he says.

"What's so neat about the time capsule is that these were things that were specifically chosen to share with us in the unknown future," he adds. "It's not really the items, it's the act of sharing with the future that's so interesting."

The Boston time capsule wasn't the only one opened this week. On Wednesday, a time capsule sealed 100 years ago by Wall Street businessmen was opened at the New-York Historical Society. The bronze chest contains financial reports and commercial directories and was supposed to be opened in May 1974. But the capsule was forgotten, before it was found again in the 1990s and deemed the oldest unopened time capsule in the world. That is, until the discovery of the Boston capsule.

Student historians at the New-York Historical Society are already assembling items for their own time capsule to be opened in 2074. They include an Amazon Kindle, a gay pride T-shirt, and a New York Public Library card, according to The New York Times.

When refurbishment of the Old State House lion is done, perhaps in the spring, the society will return it to its place with a new time capsule, Mr. Sheidley says. The Bostonian Society hopes Mayor Marty Walsh will write his own letter for the capsule.

The Society is also crowdsourcing suggestions for other items to include.

"The preponderance of suggestions is that we should just include a mobile device which, after all, contains all the information anyone needs today," he says with a laugh. He pauses, before adding, "But we'd need to include a charger, too."